Accelerometer

ABSTRACT

An accelerometer wherein a proof mass is adapted for movement with respect to a coordinate axis in response to acceleration along the axis. A signal is generated in response to the movement for providing a readout proportional to acceleration, and a force is generated on said proof mass in response to the signal in a manner to realign the proof mass. The amount of force can be regulated per unit of signal, and the alignment of the movable member may be adjusted.

United States Patent [191 [111 3,789,672 Davies Feb. 5, 1974ACCELEROMETER 3,438,266 4/1969 Carow et a1 73/516 R [75] Inventor: JamesW Davies, Wayne NJ. 3,438,265 4/1969 Davies et a1. 73/516 R [73]Assignee: Singer-General Precision, lnc.,Little primaw Examiner Jan1e Jin Falls, NJ- Attorney, Agent, or FirmThomas W. Kennedy [22] Filed: Nov.12, 1970 [21] Appl. No; 88,593 1 1 ABSTRACT An accelerometer wherein aproof mass is adapted for movement with respect to a coordinate axis inre- [52] US. Cl. 73/497, 73/516 R [51] Int. Cl. G0lp 15/08 sponseacceleranon along the A [58] Field at Search 73 I517 B 516 R 497 eratedin response to the movement for providing a readout proportional toacceleration, and a force is [56] References Cited generated on saidproof massin response to the signal in a manner to realign the proofmass. The amount of UNITED STATES PATENTS force can be regulated perunit of signal, and the g alignment of the movable member may beadjusted. oga et a 3,132,521 5/1964 Krupick et a1. 73/517 B 2 Claims, 3Drawing Figures |4- s I6 f "5 1s 24 F 28b I 5:2225 gfi l2 E: I as s,

Patented Feb. 5, 1974 3,789,672

2 Sheets-Sheet 1 INVENTOR JAMES w. DAVIES Patented Feb. 5, 19743,789,672

2 Sheets-Sheet 2 5 n. N INVENTOR JAMES w. DAVIE BY i flwchlal {Q6431 iiiM [6.0 '1" Mun? A ORNEY 1 ACCELEROMETER BACKGROUND OF THE INVENTION Thisinvention relates to an instrument for measuring acceleration, and moreparticularly to a linear axis acceleormeter for providing an electricaloutput signal in proportion to input acceleration.

Single axis accelerometers are well known and normally utilize apendulous mass, or proof mass, carrying a displacement transducer andelectromagnetic torquer, and adapted for pivotal movement along one axisin response to acceleration. The mass moves between a pair of spacedmagnets carrying the fixed elements of the transducer and generates anoutput signal which activates the torquer and causes the pendulous massto return to a null position. This signal passing through the circuit isproportional to acceleration.

However, in these arrangements the output signal is often not linear dueto the varying mechanical and magnetic properties of the magnets andtheir associated pole pieces and coils. Also, there is no means ofprecisely setting or adjusting the scale factors of the accelerometersand for compensating for transient changes in scale factors due tochanges in environmental conditions. Further, in these arrangements acombination of mechanical and electrical trimming is required toprecisely set the restraint or bias associated with accelerometers.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide an accelerometer of the above type which can beadjusted so that the output signal is linearly proportioned to inputaccelerations.

It is a further object of the present invention to provide anaccelerometer of the above type in which the scale factor of theaccelerometer can be adjusted.

It is a further object of the present invention'to provide anaccelerometer of the above type in which the restraint on the proof masscan be adjusted mechanically to a precise degree.

Toward the fulfillment of these objects, the accelerometer of thepresent invention comprises a movable member mounted in said casing,reference means for establishing a reference position for said movablemember with respect to a coordinate axis, said movable member beingadapted to move with respect to said reference means in response toaccelerations along said axis, means to generate a signal in proportionto said movement, means for establishing a force on said movable memberin proportion to the magnitude of said signal in a manner to realignsaid movable member relative to said reference means, means to regulatethe amount of force per unit of said signal, and means to adjust thealignment of said movable member relative to said reference means.

BRIEF DESCRIPTION OF THE DRAWINGS Reference is now made to theaccompanying drawings for a better understandingof the nature andobjects of the present invention. The drawings illustrate the best modepresently contemplated for carrying out the objects of the invention andare not to be construed as restrictions or limitations on its scope. Inthe drawings:

FIG. 1 is a top plan view of the accelerometer of the present invention;

FIG. 2 is a cross-sectional view taken along the line 2-2 of FIG. 2; and

F IG. 3 is a cross-sectional view taken along the line 33 of FIG. 1.

. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2 ofthe drawings, the accelerometer of the present invention comprises anupper housing 10 and a lower housing 12 having apertured flanges forconnection to a platform, a vehicle, or the like. As shown in FIG. 2,the lower housing 12 telescopes over the upper housing 10, and an O-ringseal 14 is provided therebetween.

A proof mass, in the form of a pendulum formed by a flat circular plate16 of a relatively thin thickness having a flange 18 extending aroundits outer circumference, is mounted in the housing by means of aonepiece flexure hinge assembly consisting of a cylindrical portion 20having one end abutting the bottom portion of the upper housing 10, anecked-down portion 22 integral with the cylindrical portion 20, andaconnection member 24 connecting the necked-down portion 22 to the flange18. A mounting screw 26 threadedly engages in the upper housing, withits head abutting the cylindrical portion 20 of the flexure hingeassembly in order to mount the pendulum 16 in the housing.

A pair of coiled electrical conductors 28a and 28b are wrapped aroundthe outer circumference of the flange l8, and a pair of disk-likeflanged pole pieces 30 and 32 are mounted in the housing immediatelyadjacent the pendulum 16. The pole pieces 30 and 32 are magneticallyattracted to, and in abutment with, a pair of magnetic rings 34 and 36extending between the pole pieces 30 and 32 and a pair of magnetic coverplates 38 and 40, respectively.

A magnetic gap is established between the flanged portions of the polepieces 30 and 32 and the corresponding inner surfaces of the coverplates 38 and 40, with a pair of non-magnetic spacer rings 42 and 44filling portions of the gaps.

A conductive coating is provided on both surfaces of the pendulum 16 inorder to form two capacitor plates shown in general by the referencenumerals 50 and 52. Each of these plates cooperates with the pole pieces30 and 32, respectively, to form an electrical capacitor in aconventional manner.

A pair of shunt screws 54 and 56 are provided through openings formed inthe cover plates 38 and 40, respectively, and are in threaded engagementtherewith so that their axial position may be regulated in order toregulate the flux path established by the magnetic rings 34 and 36 viathe pole pieces 30 and 32 and the cover plates 38 and 40, respectively.

A pair of temperature compensating sleeves 58 and 60 extend in the innerwall of the magnetic rings 34 and 36 for shunting a portion of themagnetic flux from the rings for reasons that will be explained indetail later.

With reference to FIG. 3, the flange 18 of the pendulum 16 has afinger-like extension 18a extending therefrom which carries a permanentmagnet 62. A magnetic steel adjustment screw 64 is threadedly engaged inthe upper housing 10 and carries a pair of permanent magnets 66 and 68,each extending perpendicular to the magnet 62. The adjustment screw 64is rotatable with respect to the upper housing, so that differentdegrees of magnetic flux interaction between each of the magnets 66 and68 and the magnet 62 can be achieved in order to control the magneticattraction between the screw and the pendulum 16 thereby preciselycompensating for the restraint on the pendulum, exclusively bymechanical means.

In operation, the pendulum 16 pivots about the necked-down portion 22 inresponse to accelerations in either direction along a vertical axis asviewed in FIG. 2. This movement varies the capacitance of each of thecapacitors formed by the plates 50 and 52 and their corresponding polepieces 30 and 32, respectively, which signal is directly proportional toacceleration and can be directly read out. The torquer coils 28a and 28bare also connected in a circuit with the abovementioned capacitors, sothat the above movement changes the current level in the torquer coilsand therefore the interaction between the magnetic flux emitted by thelatter and the magnetic rings 34 and 36 at the gap between the flangedportions of the pole pieces 30 and 32 and the corresponding portions ofthe cover plates 38 and 40. The design is such that the abovementionedinteraction returns the pendulum 16 to a null position or a positionmidway between the pole pieces 30 and 32, which interaction can beprecisely controlled by means of the shunt screws 54 and 56. Also, thebias or restraint on the pendulum 16 can be precisely adjusted forcalibration purposes by means of rotating the magnetic adjustment screw64 to vary the magnetic reaction between the magnets 66 and 68 and themagnet 62.

In the event of a change in the magnetic flux of the magnetic rings 34and 36 as a result of temperature changes, which would normally upsetthe calibration of the instrument, the temperature compensating sleeves58 and 60 come into play to compensate for the flux change. As statedabove, the sleeves 58 and 60 are positioned in a manner to normallyshunt a portion of the magnetic flux from the magnets 34 and 36, and thematerial of the sleeves is selected so that the degree of shuntingvaries with varying temperatures to compensate for variations in themagnetic flux from the magnets as a result of temperature changes. Forexample, the amount of magnetic flux from the magnets 34 and 36decreases in response to increases in temperature, while the material ofthe sleeves 58 and 60 can be selected so that the amount of flux shuntedby the sleeves decreases in response to the increase in temperature.Therefore, the net effect is a stable flux density in the magnetic gapsbetween the flanged portions of the pole pieces 30 and 32 and thecorresponding portions of the cover plates 38 and 40.

Of course, other variations of the specific construction and arrangementof the accelerometer disclosed above can be made by those skilled in theart without departing from the invention as defined in the appendedclaims.

I claim:

1. An accelerometer comprising a casing having a sensing axis, apendulum mounted in said casing coaxially therewith, said pendulum beingadapted to move with respect to said casing in response to accelerationsalong said axis, first and second capacitive pickoff units mounted onsaid pendulum coaxially therewith to generate a signal in proportion tosaid movement, first and second torquer units disposed adjacent saidrespective pickoff units coaxially therewith for establishing first andsecond variable forces on said pendulum, said forces varyingsubstantially in direct proportion to the magntidue of said signal, saidforces providing a net force-to-movement ratio having a selective scalefactor, first and second manual torque adjusting units for adjustingsaid torquer scale factor before start-up, first and second automatictemperature responsive torque adjusting units for automaticallyadjusting said torquer scale factor after start-up, and bias means toadjust the alignment and null position of said pendulum before start-up,wherein said casing has first and second axially spaced end walls and acylindrically shaped peripheral wall forming a cylindrically shapedsealed cavity containing a fluid and symmetrically disposed about saidsensing axis, and wherein said pendulum has a flat circular web plateportion and has a cylindrically shaped outer flange ring portionsymmetrically disposed about said sensing axis, wherein said first andsecond capacitive pickoff units respectively include first and secondcapacitor plates respectively mounted on axially opposite faces of saidpendulum web plate portion, wherein said first and second capacitivepickoff units have means to establish an electrical circuit includingsaid capacitor plates, wherein said end walls respectively havesubstantially fiat axially inner faces, said flat inner wall faces beingsubstantially parallel and being separated by a space of substantiallyuniform thickness, and wherein said first and second capacitor platesrespectively have substantially fiat axially outer faces respectivelyfacing said flat inner wall faces, said flat outer plate faces beingsubstantially parallel to and being separated from said flat inner wallfaces respectively forming first and second gaps having selectivethicknesses whereby the ratio of gap thickness to plate area isminimized.

2. The accelerometer of claim 1 wherein said first and second manualtorque adjusting units respectively include first and second shuntscrews respectively mounted on said first and second casing end wallsand respectively disposed on axially opposite sides of said pendulum webportion coaxially therewith, and wherein said first and second automatictorque adjusting units respectively include first and second sleevemembers respectively mounted on said first and second casing end wallsand respectively disposed on axially opposite sides of said pendulum webportion coaxially therewith and respectively surrounding said first andsecond shunt screws.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,789,672 I Dated Fe r y 5, 971;-

Inventor(s) James W.. Davies It is certified that error appears intheQabove-iden'tified patent and that said Letters Patent are herebycorrected as sho'wn'below:

On the cover sheet, in the heading, itemK'YB] should read as follows:

-- Assigneet Singer Company, Little Q Falls, N. J

Signed and sealed this lSthg day of June. 197M (SEAL) Attest: I I

EDWARD M.FL THER,JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM powso uscoMM-Dc 60316-P69 i ".5. GOVERNMENT PRINTINGOFFICE: "l9 0-866-33L

1. An accelerometer comprising a casing having a sensing axis, apendulum mounted in said casing coaxially therewith, said pendulum beingadapted to move with respect to said casing in response to accelerationsalong said axis, first and second capacitive pickoff units mounted onsaid pendulum coaxially therewith to generate a signal in proportion tosaid movement, first and second torquer units disposed adjacent saidrespective pickoff units coaxially therewith for establishing first andsecond variable forces on said pendulum, said forces varyingsubstantially in direct proportion to the magntidue of said signal, saidforces providing a net force-to-movement ratio having a selective scalefactor, first and second manual torque adjusting units for adjustingsaid torquer scale factor before start-up, first and second automatictemperature responsive torque adjusting units for automaticallyadjusting said torquer scale factor after start-up, and bias means toadjust the alignment and null position of said pendulum before start-up,wherein said casing has first and second axially spaced end walls and acylindrically shaped peripheral wall forming a cylindrically shapedsealed cavity containing a fluid and symmetrically disposed about saidsensing axis, and wherein said pendulum has a flat circular web plateportion and has a cylindrically shaped outer flange ring portionsymmetrically disposed about said sensing axis, wherein said first andsecond capacitive pickoff units respectively include first and secondcapacitor plates respectively mounted on axially opposite faces of saidpendulum web plate portion, wherein said first and second capacitivepickoff units have means to establish an electricaL circuit includingsaid capacitor plates, wherein said end walls respectively havesubstantially flat axially inner faces, said flat inner wall faces beingsubstantially parallel and being separated by a space of substantiallyuniform thickness, and wherein said first and second capacitor platesrespectively have substantially flat axially outer faces respectivelyfacing said flat inner wall faces, said flat outer plate faces beingsubstantially parallel to and being separated from said flat inner wallfaces respectively forming first and second gaps having selectivethicknesses whereby the ratio of gap thickness to plate area isminimized.
 2. The accelerometer of claim 1 wherein said first and secondmanual torque adjusting units respectively include first and secondshunt screws respectively mounted on said first and second casing endwalls and respectively disposed on axially opposite sides of saidpendulum web portion coaxially therewith, and wherein said first andsecond automatic torque adjusting units respectively include first andsecond sleeve members respectively mounted on said first and secondcasing end walls and respectively disposed on axially opposite sides ofsaid pendulum web portion coaxially therewith and respectivelysurrounding said first and second shunt screws.